This invention relates to a torque split control apparatus for use with a 4WD vehicle supported on a pair of primary drive wheels and a pair of secondary drive wheels.
For example, Japanese Patent Kokai No. 4-103433 discloses a torque split control apparatus for use with a 4WD vehicle. The conventional torque split control apparatus is arranged to transmit the drive from the engine to the primary drive wheels of the vehicle and to the secondary drive wheels of the vehicle through a torque distributing clutch capable of varying a torque transmitted to the secondary drive wheels. A difference between the diameters of the tires of the primary and secondary drive wheels is sensed to calculate a desired torque transmitted through the torque distributing clutch to the secondary drive wheels. A dead zone is calculated as a function of tire diameter difference and vehicle speed. The calculated dead zone increases as the sensed tire diameter difference increases and as the sensed vehicle speed increases. The calculated dead zone is subtracted from the calculated wheel speed difference to correct the wheel speed difference. A clutch torque is calculated based upon the corrected wheel speed difference. The torque distributing clutch is controlled to transmit the calculated clutch torque to the secondary drive wheels.
With the conventional torque split control apparatus, the dead zone set as a function of tire diameter difference and vehicle speed retains the clutch torque at a smaller value than obtained with no tire diameter difference. However, the clutch torque increases with no limitation as the tire diameter difference increases and as the vehicle speed increases to damage the benefits of good transfer and differential durability, minimized vibration and fuel economy.